Medical imaging systems may use nuclear materials, called radiopharmaceuticals, for imaging. First, an organ of interest to be imaged is identified. Then, pharmaceuticals are selected that will preferentially accumulate in that organ of interest. Radioactive tracers are attached to those pharmaceuticals, then administered to the patient. Radioactive tracers collect in the organs, then decay, emitting gamma radiation. Detectors such as gamma cameras are sensitive to gamma radiation to convert the radiation into corresponding electrical pulses.
Gamma cameras may be used in planar imaging. This imaging is also used in single photon emission computed tomography, or SPECT. In SPECT imaging, many projection images may be acquired from different projection angles, which are then used to reconstruct a three-dimensional model of the organ of interest.
One problem in nuclear imaging is photon scatter. Scatter results when waves and particles collide, deflecting waves from their original path. Consequently, waves may differ from their original direction of propagation, energy, and phase. In Compton scattering, the gamma rays collide with electrons in body tissue. The incident photon loses some of its energy, and is deflected from its original path. In medical imaging without scatter correction it is assumed that the detected photons traveled in a direct path from the source of radiation to the detector surface without accounting for the deflections. Therefore, scatter artifacts are left because the deflected photons are restored to the wrong points of origin. Consequently, the reconstructed image contains artifacts that increase the background and reduce contrast.
Image contrast improvement is important in several applications. For example, in planar imaging of tumors, high tumor to background contrast is desired. In SPECT oncology, high tumor to background contrast is critical for tumor detection. In cardiac studies, accurate cardiac wall to chamber contrast is important for accurate ejection fraction quantization. An accurate perfusion image of the myocardium wall is also critical for diagnosis and prognosis. Scatter can significantly reduce the cardiac wall to chamber contrast and can make an under-perfused myocardial wall appear normal.
Techniques have been developed for scatter correction. A first technique models scatter based on data acquired in energy windows outside of the photon peak window. The image is then adjusted to account for the modeled scatter. However, this technique requires acquisition of additional data that increases the complexity of the data acquisition software, the acquisition setup, and also the management of the patient database. Furthermore, the accuracy of the technique is also limited by the accuracy of the model, and the amount of noise acquired in the additional scatter windows.
A second technique estimates the scattered photons based on physics models that model the Compton scatter process. Some techniques use Monte Carlo (or pseudo Monte Carlo) simulation in the iterative reconstruction to estimate photon scatter. The accuracy of these techniques is limited by the accuracy of the physics model and the accuracy of the attenuation maps. Furthermore, the technique can be very computationally expensive.